Abstract This present paper describes an experimental study focused on the influence of the interface strength, between the lightweight aggregate concrete (LWAC) and the ultra-high durability concrete (UHDC), on the structural behavior of composite beams. The beams were produced with a new concept which consists of using an UHDC layer only in the cover, being the core produced with LWAC. The great benefit of using this ‘superskin’ is to apply UHDC only where it is greatly needed for durability purpose and not in the total cross section of the structural members, avoiding high cement consumption, which is important to reduce the environmental impact associated to cement production. Thirteen different beams were tested until bending – or shear – controlled failure, varying the type of interface between the LWAC core and the UHDC cover: i) smooth; b) rough; c) with steel connectors; and d) indented. Several slant shear tests (SST) were performed to evaluate the longitudinal shear strength of the LWAC-UHDC interfaces. The main goals of this study were: 1) to analyze if composite beams’ behavior is monolithic under load increasing, in both flexural an shear tests, i.e., to check for possible slip of the outer layer; 2) to evaluate the influence of the interface roughness and of UHDC cover on structural performance, and finally 3) to evaluate which is the most proper interface to produce this type of composite beams. The results show that the superskin greatly enhances the structural behavior of the beams and that an appropriate roughness is necessary at the interface between the two concretes, to make the proposed solution viable and reliable.